The present invention relates in general to digital filters, and more directly, to digital filters for use in remote control receivers, in particular, for audio-frequency centralized ripple control receivers.
Remote control receivers which receive an input signal in the form of a signal superimposed upon the line voltage, especially ripple control receivers, are used in some industries in large numbers. For reasons of economy, it is essential that such receivers be dependable and inexpensive, notwithstanding the technical requirements placed thereon. For practical reasons, it is additionally desirable to maintain the size of such receivers as small as possible, and this has been accomplished in recent years with the use of microprocessors.
Such receivers typically include a selective receiver section for the assigned remote control frequency, an evaluation section responsive to the output of the receiver section for detecting the remote control commands which are in coded form, and an output section for effecting control on the basis of the detected commands.
In conventional electronic or electromechanical remote control receivers of the type used, for example, in centralized ripple control systems, cascades of LC filters, or RC active filters of the second order, have been typically employed for the receiver-side recovery of the remote control signals or remote control commands which are delivered at the input side to the remote control receiver together with the main voltage, the harmonics thereof and possibly other interference frequencies.
In these cases, filters of this type are designed as narrow band-pass filters. The quality factor Q is usually about 30, so that a relatively-weak remote control signal of, for example, from about 1 to 8 volts may be filtered out for evaluation from the low-voltage mains of, for example, 220 V (50 Hz), which, to some extent, carries very considerable interferring signals. In this respect, see, for example, the de Vries U.S. Pat. No. 4,126,793, which is assigned to the same assignee as the present application.
The mains voltage of 220 V and the harmonics thereof having voltages of normally up to 15 volts, and occasionally even slightly higher, occur as interference frequencies. In this respect, see the publication VDEW: Empfehlung fur die Frequenzplanung bei FrequenzRundsteueranlagen, Verlags-und Wirtschaftsgesellschaft der Elektrizitatswerke mbH, 6 Frankfurt (Main) 1970.
A method of and an apparatus for the remote transmission of signals has already been proposed in the Kniel et al U.S. Pat. No. 4,007,360, also assigned to the same assignee as the present application, in which method a sampled data filter is provided in the form of a digital band-pass filter. Furthermore, an electronic ripple control receiver has been proposed in DOS No. 2,708,074, without, however, more detailed information being given about the digital filter provided therein.
In addition to the publications which have already been mentioned, reference is also made to the following prior art works: Halbleiter-Schalttechnik (Tietze/Schenk), 5th Edition, Springer-Verlag Berlin 1980; the dissertation No. 5,645/1976 ETH Zurich; Probleme der Realisierung digitaler Filter, (F. Bonzanigo) and the publication "Theory and Design of Digital Filters", Electronic Engineering, May, 1977, Morgan Grampian Limited, Galderwood Street, London SE 18 2 BP.
When a digital filter is used with a microcomputer as a processor unit for the evaluation section of a ripple control receiver, it has been difficult to provide a filter having the properties required by such intended use in an economical manner. A particular difficulty arises in that, with use of the economically-desirable 8-bit processor unit (microcomputer), a suitable digital filter of known type cannot be realized without taking special measures.
Bearing in mind the dynamic range required by such use, a known analog filter for remote control receivers cannot be readily transformed into a digital filter according to generally-known rules. Due to the internal gain of the filter in response to the input signal, i.e., the remote control signal itself, an 8-bit processor unit already tends to overflow, even when no interference signal has appeared in the input signal. With respect to the useful signal/interference signal ratios alone which occur in practice, an 8-bit processor unit would indeed have a dynamic range which would just suffice for the signal processing of the control signal alone. Thus, it has been found in practice that the signal gain which has been mentioned jeopardizes the perfect operation of the filter. The overflow within the filter stages caused thereby is also a result of the quality factor Q of about 30 which is needed for practical requirements, and is a result of the sampling ratio of at least two, which is required in practice, i.e., the ratio between the sampling frequency of the input signal and of the resonant frequency.
It is very important for the use of a digital filter in a remote control receiver, in particular, in an audio-frequency centralized ripple control receiver, that the pass-band characteristic which is necessary for perfect operation be realized with utmost economy. It will probably always be financially advantageous to be able to use a processor unit having as short a dynamic range as possible (for example, only 8 bits), such as provided by a microcomputer having an 8-bit processor unit.
By the stabilization of the clock frequency, whether, for example, by means of a known crystal oscillator or by phase locking the clock frequency to a reference frequency, for example, to the mains frequency, an adequate frequency stability may be achieved with a very low expenditure, even over a long duration of use. Therefore, the filter characteristic of a digital filter of this type may be designed within a restricted, but in practice, adequate frequency range in basically the same manner as for hitherto conventional analog filters, i.e., the same amplitude response and the same phase response is achieved as with the conventional filters. However, for the production of a filter of this type by means of a processor unit (microcomputer), for example, according to DOS No. 2,708,074, the fact that the digital filtering must be realized in the processor unit with an adequately-low expenditure in calculating time, must be taken into consideration because the system must be operated in real time.
Another objective to be aimed for is that a processor unit from the class of the most financially-desirable "one-chip" microcomputers be selected. A price-determining factor is the so-called bit number of the microcomputer. In the present case, this term is understood as meaning the length of the data words which are used, i.e., the number of binary positions per number. The values which are compiled in Table 1 are conventional at present.
TABLE 1 ______________________________________ 4 Bit Number range 0-15 (2.sup.4) Dynamic range 24 db 8 Bit Number range 0-255 (2.sup.8) Dynamic range 48 db 12 Bit Number range 0-4 095 (2.sup.12) Dynamic range 72 db 16 Bit Number range 0-65 535 (2.sup.16) Dynamic range 96 db ______________________________________
The bit number defines the processor dynamic range of the microcomputer or processor unit.
With respect to the processor speed, care must also be taken that a financially-favorable standard product be used. This means that for the digital filter, minimal structures should be used which require as low a processor expenditure as possible for the filtering, i.e., few additions, few multiplications and few memory manipulations. Such minimal requirements are provided by recursive filters in a "direct form", like those described in the publication "Theory and Application of Digital Signal Processing" (Prentice Hall International, Inc., London, 1975) by Rabbiner/Gold, on Page 41 et seq.
A man skilled in the art will transform the characteristic coefficients of an analog filter by one of the known transformation techniques, as described in the above-mentioned publication and also in the publication by Tietze/Schenk, which has already been mentioned, for example, a bilinear transformation, such that he obtains the coefficients of the digital filter with similar properties. The digital filter then conforms substantially to the analog filter with respect to its pass-band characteristic up to half the sampling frequency. He then realizes these coefficients in a suitable filter structure according to the above-mentioned references.
Although this method will achieve the purpose as regards the required pass-band characteristic, it has the disadvantage that the resulting digital filter requires a substantially greater dynamic range of the processor unit to be used than would be necessary from the given useful signal/interference signal ratio. It has been found that, as a result of the relatively-high quality factors Q which are necessary for filters in centralized ripple control uses, the individual filter stages have a correspondingly-high amplification of the resonant frequency fg (i.e., in this case of the useful signal). The correlation represented in Table 2 applies in the case of a filter stage of the second order having a quality factor Q of 30.
TABLE 2 ______________________________________ Sampling frequency f.sub.s approximate amplification at ______________________________________ fg 10 .times. f.sub.g 100 6 .times. f.sub.g 55 4 .times. f.sub.g 40 ______________________________________
The amplification v.sub.g at frequency f.sub.g is calculated according to the following formula: ##EQU1## for a filter stage of the second order wherein EQU Z.sub.i =.vertline.e.sup.j.omega.g -z.sub.i .vertline.Z.sub.i =Z.sub.i, Z.sub.2 (I)
and EQU P.sub.i =.vertline.e.sup.j.omega.g -P.sub.i .vertline.P.sub.i =P.sub.1, P.sub.2 (II)
z.sub.i are the zeros of the Z-transfer function of the filter PA1 P.sub.i are the poles of the Z-transfer function of the filter.
In this respect, also see the above-mentioned publication Rabbiner/Gold, Chapter 218.
As a tendency, it is to be established that the resonant frequency amplification v.sub.g of the filter stage also increases with an increase in the filter quality factor Q and with an increase in the sampling rate (ratio of the sampling frequency f.sub.s to the resonant frequency f.sub.g).
With a sampling rate of from about 6 to 10, and with a filter quality factor of 30, a gain of the wanted signal by factor 55 to 100 is to be reckoned with. This means that a minimal wanted signal which is reasonably given to the first filter stage with at least .+-.2 processor units (one processor unit corresponds to the number "1"), appears at the output with from .+-.110 to .+-.200 processor units. In the case of higher useful signal levels which could quite easily occur in practice, the need for a processor dynamic range of the processor unit increases accordingly, as shown by the following Table 3.
TABLE 3 ______________________________________ Arithmetic units at the filter Necessary dynamic Useful Signal Input Output range ______________________________________ 1 V .+-.2 .+-.110 to .+-.200 8 to 9 bit 2 V .+-.4 .+-.220 to .+-.440 9 to 10 bit . . 8 V .+-.16 .+-.800 to .+-.1760 11 to 12 bit ______________________________________
If it is assumed that, in existing low-voltage mains, the signal/interference voltage ratio for any interference voltage which arises over a comparatively-long period of time, for example, a mains harmonic, hardly falls below the value 1/20, then it is seen that even under circumstances in which several interference voltages occur at the same time, a processor resolution of 1/256, as given by an 8-bit processor unit, should actually suffice, if it succeeds in eliminating the internal amplification of the filter stages. However, it is assumed that the voltage of the base frequency has itself already been reduced to a sufficient extent by a suitable analog pre-filter. This is possible using simple known means and a pre-filter is necessary anyway, as is known, because of the ambiguity of digital filters.
It is therefore apparent from the foregoing discussion that the task of digital filtering of centralized ripple control signals by means of financially-desirable processor units (8-bit microcomputers) and by an integrated 8-bit-analog-digital converter in a "one-chip" design and by means of minimal filter structures, cannot be achieved by simply transforming an existing analog, narrow band-pass filter into a digital, narrow band-pass filter according to generally-known rules. As a result of an internal amplification of the useful signal, the 8-bit processor unit tends to overflow, that is, even when there are still no interference frequencies. From the view point of the wanted signal/interference signal ratios alone, an 8-bit processor unit would, however, have an adequate dynamic range.